The present invention concerns catalysts for the polymerisation of ethylene and, more particularly, Ziegler type catalysts, having a solid solution structure.
Numerous Ziegler type catalysts are already known for the polymerisation of ethylene and alpha-olefins. These catalysts generally comprise the catalytic component itself, consisting of at least one halogen compound of a transition metal of Groups IV to V of the Periodic System, and an activator selected from the hydrides and organometallic compounds of the metals of Groups I to III of the Periodic System. The catalytic component comprising the transition metal may be supported by an inert carrier, such as alumina, silica, magnesium oxide, magnesium halides etc.
The present invention relates more particularly to Ziegler type catalysts, comprising at least two halogen compounds of transition metals, one of which is a halogen compound of titanium, crystallised together with a halogen compound of a metal of Group III of the Periodic System, and the other of which is a halogen compound of a metal of Groups V b and VI b of the Periodic System, these catalysts being optionally supported by an inert carrier.
The literature gives some examples of Ziegler type catalysts comprising two halogen compounds of two different transition metals; for example, U.S. Pat. No. 3,288,769 describes catalytic compounds for polymerising ethylene in particular, having the formula (TiCl.sub.3, VCl.sub.3).sub.x, x lying between 0.25 and 4, and permitting a catalytic yield of 15 grams of polymer per milli-atom of titanium and vanadium per hour at 40.degree. C. Similarly, U.S. Pat. No. 3,223,651 describes a catalyst of formula EQU y TiCl.sub.3, (1-y) VCl.sub.3, 0.33 AlCl.sub.3,
y lying between 0.5 and 0.97, capable of producing a synergistic effect for the polymerisation of alpha-olefins at a temperature below 100.degree. C. and at a pressure below 35 bar.
On the other hand, it is khown to polymerise ethylene at a pressure of between 300 and 2500 bar, approximately, and at a temperature of between 180.degree. C. and 300.degree. C., approximately. Within the framework of this kind of process, it is desired to improve, on the one hand, the production yield, related to the catalyst used, and, on the other hand, the quality of the polymer produced by modifying the following parameters in particular: density, molecular weight and distribution of molecular weights. It is therefore an object of the present invention to develop catalysts that can be used for polymerising ethylene in conditions of high temperature and high pressure, as defined above, and capable of improving yield and quality of the polymer produced.